1. Field of the Invention
The present invention relates to a delivery system for delivering vapors of solid state precursors into a chemical vapor deposition reactor for thin film deposition.
2. Background of the Prior Art
Chemical vapor deposition has emerged as an important technique for thin film deposition in contemporary microelectronics and other areas. A large variety of materials can be obtained by this method including pure metal films, semiconductors, and dielectric. Although chemical vapor deposition of semiconductors and metals has been investigated for quite some time, only recently has chemical vapor deposition been studied for the formation of different oxide materials. Research into high temperature super conducting thin films has resulted in the application of chemical vapor deposition methods for the production of different ferroelectric, magnetoresistant, thermoelectric, and other electrical, magnetic, and optical materials.
In the past, precursor delivery in chemical vapor deposition did not pose problems. Liquid or even gaseous compounds like SiH4, AsH3 or Al(CH3)3 could be used in the deposition of a variety of semiconductors and metals. Even solid precursors such as CBr4, which is used as a p-type dopant in the growth of GaAs layers, is sufficiently stable to be evaporated from a container having a constant high temperature.
However, many new materials require less volatile and frequently less stable precursors for their deposition. For example, BaO is a common component of high temperature superconducting, ferroelectric, and other thin film oxide materials [Ba(dpm)2] (dpm=dipivaloymethane) and its derivatives are the most common compounds used to introduce Ba into the films of such materials. Unfortunately, this complex evaporates only at about 250 degrees Celsius and is prone to oligomerization during heating, which decreases its vapor pressure.
Liquid precursors are convenient for chemical vapor deposition as they allow the use of a bubbler for their evaporation at temperatures at and around room temperature. This provides a constant precursor vapor pressure during the deposition. However, for some applications, precursor compounds are not known, while for other applications, the compounds are very difficult to synthesize and frequently tend to be unstable as well as air and moisture sensitive.
Therefore, many manufactures use solid precursors for the chemical vapor deposition process. For the deposition of multicomponent materials, several precursors are needed, typically one volatile compound for each component, with each compound being evaporated from a separate container. The use of multiple containers results in a large number of process parameters (temperature and flow of carrier gas for each container) which considerably complicates the control and understanding of the overall deposition process.
Therefore, a device is needed that will simplify the overall chemical vapor deposition process by evaporating all precursors together. Many solutions have been proposed. One such method dissolves all of the necessary precursors in a suitable solvent with the solution being introduced into the hot zone of an evaporator and evaporated along with the precursors. A micropump or an ultrasonic nebulizer with an inert carrier gas can be used for delivery of the solution into the evaporator. While this method has produced successful result, it has certain disadvantages. The use of the solvent raises the possibility of film contamination from carbon resulting from the decomposed solvent which contamination is highly unfavorable in electronic applications. Additionally, the use of a micropump or nebulizer increases the costs of the process and raises the possibility of nonreproducability of the properties of the film.
Another method evaporates all precursors simultaneously by introducing them as a mixture of solid compounds. Unfortunately, particles of many precursors are apt to stick together requiring a special feeder for the introduction of the precursor powder into the evaporator. One such special feeder achieves a continues precursor supply by providing a capillary having a slit along the slide. A mixture of the precursors fills the capillary which is then slowly moved into the hot zone of the evaporator wherein the mixture evaporates. This feeder proves problematic when used with a large mass of precursors, with precursors which melt before evaporation, such as Ba(dpm)2, and with mixtures of precursors having substantially different vapor pressures. Another type of solid precursor feeder uses the flash-evaporation technique wherein a mixture of precursors is introduced into the hot zone in small portions. Each portion of mixture must evaporate completely before the next portion is introduced. While such a device can achieve desirable results, the device tends to be complicated and costly.
Therefore, there is a need in the art for a precursor delivery system capable of delivering a powder precursor into an evaporator, which system overcomes the above-stated problems in the art. Such a system, which must be relatively simple in design and construction and be relatively inexpensive to manufacture and operate, must be able to deliver most types and mixtures of powder precursors into the evaporator. The system must minimize the potential for contamination of the film being produced and must allow for a high degree of reproducibility of the properties of the film being produced.
The powder precursor delivery system for chemical vapor deposition of the present inventions addresses the aforementioned needs in the art. The system, which is relatively simple in design and construction and is relatively inexpensive to manufacture and operate, is able to deliver most types and mixtures of powder precursors into the evaporator. The system minimizes the potential for contamination of the film being produced and allows for a high degree of reproducibility of the properties of the film being produced.
The powder precursor delivery system of the present invention is comprised of a housing having an open first top and an open first bottom. A container having a second open top and a second open bottom is disposed within the housing while a roller having a nonstick outer surface and a circumferential groove, is rotatably disposed within the housing and positioned such that a portion of the outer surface faces the second bottom of the container. A precursor is placed into the container as is a plurality of beads wherein the precursor coats the beads. The beads exit the container by being transported within the groove of the roller as the roller is rotated and eventually dropping out of the groove by the force of gravity. The beads are made from an inert material that does not react with the precursor such as stainless steel. The precursor is placed into the container in either powder or crystal form. A motor is operatively connected to the roller for rotating the roller. An evaporator having a open third top attached to the first bottom of the housing and a third bottom is located below the container. A first inlet tube is attached to the evaporator for introducing a first gas into the evaporator. A first screen is disposed within the evaporator while a second screen is disposed within the evaporator downstream of the first screen. An outlet tube extends between the third bottom of the evaporator and a reactor for fluid flow connecting the evaporator with the reactor. A second inlet tube is attached to the outlet tube for introducing a second gas into the outlet tube. A substrate holder is disposed within the reactor while a thermocouple is attached to the holder.